Bio-adsorption of heavy metals from aqueous solution using the ZnO-modified date pits

The bio-adsorption of heavy metals (including Cu2+, Ni2+, and Zn2+) in aqueous solution and also in an industry wastewater using the ZnO-modified date pits (MDP) as the bio-adsorbent are investigated. The fresh and used bio-adsorbents were characterized by FT-IR, SEM, BET, and XRD. The bio-adsorption parameters (including the pH of solution, the particle size of MDP, the shaking speed, the initial concentration of heavy metals, the dosing of MDP, the adsorption time, and the adsorption temperature) were screened and the data were used to optimize the bio-adsorption process and to study the bio-adsorption isotherms, kinetics, and thermodynamics. Two adsorption models (Langmuir isotherm model and Freundlich isotherm model) and three kinetic models (pseudo-first-order model, pseudo-second-order model, and intra-particle diffusion model) were applied to model the experimental data. Results show that the maximum adsorption amount of Cu2+, Ni2+, and Zn2+ on a complete monolayer of MDP are 82.4, 71.9, and 66.3 mg g−1, which are over 4 times of those of date pits-based bio-adsorbents reported in literature. The bio-adsorption of heavy metals on MDP is spontaneous and exothermic, and is regulated by chemical adsorption on the homogeneous and heterogeneous adsorption sites of MDP surface. This work demonstrates an effective modification protocol for improved bio-adsorption performance of the date pits-based bio-adsorbent, which is cheap and originally from a waste.


Bio-adsorption of heavy metals from aqueous solution and real wastewater
Bio-adsorption of heavy metals (including Cu 2+ , Ni 2+ , and Zn 2+ ) from aqueous solution was performed in a batch system using MDP as the adsorbents.The metal salts namely copper sulfate pentahydrate (CuSO 4 •5H 2 O), cobalt(II) nitrate hexahydrate (Ni (NO 3 ) 2 •6H 2 O), and zinc sulphate heptahydrate (ZnSO 4 •7H 2 O) were dissolved in 50 mL water in a 100-mL flask, followed by adding the adsorbents under shaking.The adsorption conditions were: pH of 2-8, adsorbent dosing of 0.4-6 g L −1 solution , adsorbent particle size of 200-1500 µm, shaking speed of 150-400 rpm, adsorption time of 10-180 min, initial concentration of heavy metals of 10-90 mg L −1 , and adsorption temperature of 25-55 °C.The liquid samples were taken at different adsorption time intervals and were analyzed on a GBC 932 atomic absorption spectrometer after the filtration using cellulose nitrate membrane with pore size of 0.45 µm.
After the bio-adsorption, the sludge (namely the spent MDP bio-adsorbent) was regenerated and reused to evaluate the reusability of the MDP.The spent MDP containing the adsorbed heavy metals was added to 14 ml aqueous solution containing 50 mM H 2 SO 4 .The slurry was stirred at a speed of 350 rpm and at a desorption temperature of 25 °C continuously for 48 h.After the filtration and washing with water, the MDP was reused for the bio-adsorption for 4 times.
In addition, a real wastewater, which was collected from an electroplating company in Baghdad and contained 21.6 mg L −1 of Cu 2+ , 13.9 mg L −1 of Ni 2+ , and 18.9 mg L −1 of Zn 2+ , was applied to evaluate the bio-adsorption performance.
The removal efficiencies of heavy metals (R, %) and the adsorption capacity of the bio-adsorbent (q e , mg g −1 ) 47,48 were calculated using Eqs.(1) 48 and (2) 49 , in which C 0 , C t , and C e are the initial concentration, the concentration at adsorption time (t, min), and the equilibrium concentration of heavy metals in the aqueous solution (mg L −1 ), respectively; v is the volume of the solution (L); and w is the weight of the bio-adsorbent (g).All the experiments were triplicated and the averaged data are reported. (1) (2) q e = (C 0 − C e ) × v w

Characterization of the bio-adsorbents
The specific surface area of Date pits (DP) is 82.4 m 2 g −1 .It consists of cellulose, hemicellulose, and lignin 50 .
The FT-IR spectrum of DP (Fig. 2a) shows the typical adsorption bands of these biomass components, e.g., at 3500-3400 cm −1 (vibration of inter-or intramolecular hydrogen bonding (O-H) in cellulose 51 ), 3000-2800 cm −1 (stretching of aliphatic C-H bonding 52 ), 1745-1600 cm −1 (stretching of unconjugated C=O bonding in hemicellulose 53 ), 1604.77cm −1 (stretching of aromatic C=C bonding 53 ), 1400-800 cm −1 (deformation of C-H bonding in lignin 54 ), 1365 cm −1 (deformation of C-H bonding in cellulose and hemicellulose 52 ), 1246 cm −1 (stretch of C-O bonding in lignin and xylan 52 ), 1083.99 cm −1 (stretch of C-O bonding in hemicellulose 52 ), and 869 cm −1 (stretch of C-O bonding in cellulose 52 ).Besides, the XRD pattern of DP (Fig. 3a) also shows the representative diffraction peaks of crystalline cellulose (2θ = 16.2 and 23.9°5 5 ), amorphous hemicellulose (2θ between 30 and 50°5 6 ), and lignin (2θ = 64.4°5 7).SEM image of DP is shown in Fig. 4a, indicating its rough surface with macropores 58 .The morphological characteristics of DP are changed after the incorporation of ZnO on DP (Fig. 4b), showing additional particles on the DP surface.EDX analysis of ZnO-modified date pits (MDP, Fig. S2) confirms the presence Zn and O, which are related to the ZnO particles.According to the EDX results, the ZnO content on MDP is about 35 wt.%.MDP contains ZnO particles with a particle size distribution of 0.1-1.2μm (centered at 0.64 μm, Fig. S1, estimated by using the Digimizer program) and have a coarse (or unsmooth) surface (Fig. 4b).Compared to DP, MDP has a higher surface area of 195.89 m 2 g −1 , which is likely related to the loaded ZnO particles on the surface and is advantageous for the bio-adsorption (vide infra) 59 .It needs to be noted that MDP was treated at 350 °C, at which temperature the main components of DP could be partially converted, e.g., lignin starts to degrade at 200 °C, hemicellulose starts to decompose at 220 °C, and cellulose starts to depolymerize at 310 °C60 .Since the thermal treatment temperature is higher than biomass torrefaction (e.g., 250-320 °C) but lower than the pyrolysis temperature (e.g., 450-600 °C), a bio-char-type material was likely obtained (Fig. 1).This is well reflected by the FT-IR spectrum of MDP (Fig. 2b, showing the disappearance and shifting of the adsorption bands for several functional groups on DP) and the XRD pattern of MDP (Fig. 3b, showing the disappearance of the diffraction peaks for the crystalline components in DP).The incorporation of ZnO onto DP is evidenced by the two characteristic adsorption bands of ZnO at 594 and 478 cm −1 (corresponding to the inorganic ZnO stretching 61 ) in the FT-IR spectrum of MDP (Fig. 2b 3b).ZnO crystal size was calculated using Scherrer equation (Eq. 3) 62 , in which K (= 0.9) is the Scherrer constant, λ (= 0.15406 nm) is the wavelength of the X-ray beam used, β is the full width at half maximum (FWHM) of the peak, and θ is the Bragg angle.The estimated ZnO crystal size is 7.0 nm, which is by far lower than the that of the particle size observed by SEM (vide supra).This indicates that the ZnO nanocrystals agglomerated on the surface of the date pits.After simultaneous bio-adsorption of heavy metals with an initial concentration of 90 mg L −1 at 25 °C and pH of 5 for 180 min, the morphological characteristics of the spent MDP (Fig. 4-) show distinct change to those of the MDP (Fig. 4-b), most likely related to deposition of the adsorbed heavy metal ions (including Cu 2+ , Ni 2+ , and Zn 2+ ) on the spent MDP surface.ZnO is retained on the spent MDP according to the characteristic adsorption bands of ZnO in the FT-IR spectrum of the spent MDP (Fig. 2c) and diffraction peaks of ZnO in the XRD pattern of the spent MDP (Fig. 3c).The average ZnO particles on the spent MDP (Fig. 4c) is about 0.64 μm, comparable to that on the fresh MDP (vide supra).These characteristics indicate a good stability of MDP for the bio-adsorption of heavy metals.

Optimization of bio-adsorption parameters
Four bio-adsorption parameters, namely the pH of the solution, the particle size of the bio-adsorbent, MDP dosing, and the shaking speed, were screened to optimize the bio-adsorption performance.For this, the bioadsorption experiments were performed for 180 min, assuming that the bio-adsorption reached equilibrium and a maximum removal of heavy metals from the solution was obtained.
Effect of pH of solution.The amount of removed heavy metals (Cu 2+ , Ni 2+ , and Zn 2+ ) by the bio-adsorption on MDP increases with the increase of the pH value from 2 to 5, followed by a decrease when the pH value is further increased, e.g., to 8 (Fig. 5a).This is likely due to that the varied pH of the solution of the adsorption system changes the solubilities of heavy metal ions, the concentrations of the counter ions on the adsorbent, and the degree of ionization of the adsorbent 63 , affecting the electrostatic interactions between positively charged heavy metal ions and negatively charged functional groups of the bio-adsorbent (such as −OH, −COOH, -O-, and −CO-NH-), which affects the adsorption capacity.At a low pH value, MDP has a positive surface charge.However, the bio-adsorbent is surrounded by hydronium ions (H 3 O + ) with a high concentration in solution, leading to a repulsive force 22 that hinders the access of the heavy metal ions to the adsorption sites of the bio-adsorbent.
With the increase of pH, the surface of bio-adsorbent becomes less positive charge 64 and may become neutral at a pH pzc (point of zero charge) of ca.6.1, which is estimated according to the experimental data shown in Fig. 5a.At a high pH value (e.g., > pH pzc ), the heavy metal ions may react with the OH − ions with a high concentration in solution 17 , which is detrimental for the bio-adsorption performance.
Effect of particle size of MDP.Removal of the three heavy metals by the bio-adsorption on MDP increases with the decrease of the bio-adsorbent particle size from 1500 to 200 μm (Fig. 5b).This could be related to the more amount of accessible adsorption sites on MDP, which were inside the big bio-adsorbent particles but are exposed to the external surface of the bio-adsorbent with a small particle size after crushing and grinding 38 .A further decreased particle size (< 200 μm) was not investigated, considering the significant difficulty of the separation of the fine bio-adsorbent particles from the solution.
Effect of shaking speed.Bio-adsorption of the three heavy metals on MDP increases with the increase of the shaking speed from 150 to 300 rpm (Fig. 5c), likely related to the enhanced external mass transfer of the metal ions from the solution to the active adsorption sites.Nevertheless, a further increasing the shaking speed from 300 to 400 rpm leads to a slight drop in the bio-adsorption performance.This might be attributed to the desorption of those heavy metals with a weak adsorption on the bio-adsorbent 65 .
Effect of MDP dose.It can be seen from Fig. 6 that the bio-adsorption efficiencies for the removal of Cu 2+ , Zn 2+ , and Ni 2+ increase proportionately with the increase of MDP dose from 0.4 to 2 g L −1 solution .This could be rationalized by the increased number of adsorption sites on the bio-adsorbents with a higher concentration in the solution.However, a further increasing the MDP dose, e.g., from 2 to 6 g L −1 solution , does not show visible increase in the removal of the three metals (Fig. 6).On one hand, increasing the adsorbent concentration could cause the aggregation/agglomeration of adsorbent particles, leading to the dead points between the adsorbent particles that are not accessible for the adsorption 59 .On the other hand, the interaction/collision of the MDP particles with a high concentration in solution could lead to the desorption of the metal ions that weakly adsorbed on the adsorbent surface 25 .As such, an optimal MDP dose of 2 g L −1 solution is determined for the present study, under which conditions the removal percentages of 97.4%, 96.7%, and 90.2%, respectively, were obtained for the bio-adsorption of Cu 2+ , Ni 2+ , and Zn 2+ using MDP (Fig. 6).
Effect of the adsorption time.In a batch bio-adsorption system, the removal percentage of heavy metals (Cu 2+ , Ni 2+ , and Zn 2+ ) on MDP increases with the adsorption time (in the initial 10-60 min, after which plateau adsorption was reached, Fig. 7).This could be related to the fewer adsorption sites available on MDP and the decreased concentrations of heavy metals in solution with the adsorption time 66 .The latter is also evidenced by the higher efficiencies for removing heavy metals with higher concentrations shown in Fig. 7.
Effect of initial concentrations of heavy metals.The effect of initial concentrations of the three heavy metals (Cu 2+ , Ni 2+ , and Zn 2+ , Fig. 7) indicates that the removal percentage increases with increased initial concentration.This is very likely related to the increased driving force at a higher concentration's gradient 67 .Effect of adsorption temperature.Increasing the bio-adsorption temperature (e.g., from 25 to 55 °C) has a negative effect on the removal of heavy metals (Fig. 8).One of the reasons could be due to the decreased viscosity of the aqueous solution at a higher temperature, resulting in an enhanced diffusion resistance to the bulk (external) and pore (internal) borders of the MDP particles 41 .Another reason might be related to the desorption of heavy metals from the adsorbent surface, which is favored at a higher temperature 68 .

Isotherm study
In literature, Langmuir 69 and Freundlich 70 adsorption models have been widely used for modelling the adsorption of heavy metals on various adsorbents 40 .In this study, the experimental data of the isothermal bio-adsorption equilibrium of the three heavy metals (Cu 2+ , Ni 2+ , and Zn 2+ ) on MDP were also simulated using the above two models (Eqs.4-6 71,72 ) and the non-linear fitting curves are shown in Fig. S3.The constants of the two models and the correlation coefficients were calculated using nonlinear regression and are shown in Table 1.
The fitting curves (Fig. S3) and parameters (R 2 and SSR, Table 1) indicate that the bio-adsorption of Cu 2+ , Ni 2+ , and Zn 2+ on MDP adjust better to the Freundlich isotherm model compared to the Langmuir isotherm model.This suggests 73 that these heavy metals were adsorbed on the multi-layer and heterogeneous adsorption sites on the MDP surface.The maximum equilibrium adsorption amounts of Cu 2+ , Ni 2+ , and Zn 2+ on a complete monolayer of MDP (q m ) are 82.4,71.9, and 66.3 mg g −1 , respectively, according to the Langmuir isotherm model (Table 1).These q m values are considerably higher than those reported in literature for the bio-adsorption of heavy ( 4) Table 1.Parameters of the two isotherm models for the bio-adsorption of heavy metals on MDP.# SSR = ∑(q e cal − q e exp ) 2 /N.SSR (Sum of squares due to regression) is the sum of the differences between the predicted value and the experimental data divided by N (total number of experiments).2, 38,74,75 ) and also those for the roasted DP (without modification of ZnO, Tables 2), likely attributed to the modified characteristics of the DP upon loading of ZnO and the followed thermal treatment (vide infra).

Kinetic study
The experimental kinetic data (Sections "Effect of the adsorption time" and "Effect of initial concentrations of heavy metals") were modeled to study the bio-adsorption kinetics, considering the practical implementation of a bio-adsorption system and also aiming to obtain insights into the types and mechanisms of the bio-adsorption of heavy metals on MDP.Two kinetic models, namely pseudo-first-order (Eq.( 7) 76 ) and pseudo-second-order (Eq.( 8) 77 ) that were widely used to model the adsorption of inorganic and organic matters from aqueous solution in literature 78,79 , were applied in this study.The parameters and the regression coefficients were calculated using the nonlinear fitting algorithms in the MATLAB program and are shown in Table 3.The good R 2 values for the pseudo-second-order model indicate that pseudo-second-order model fits the bio-adsorption kinetics better than pseudo-first-order model.
In addition, the above experimental kinetic data were also analyzed using the intra-particle diffusion model (Eq.( 9) 80 ), which is often used to decide whether intra-particle diffusion is the rate-limiting step.It can be seen from Fig. 9 that the good linear fitting plots could be obtained, however, all the C values (in Eq. ( 9)) are greater than zero.These results suggest that the intra-particle diffusion mechanism is not dominant and very likely, there are also other mechanisms such as film, surface, or pore diffusion regulating the adsorption kinetics 81 .

Thermodynamic study
The experimental data of the effect of adsorption temperature (Section "Effect of adsorption temperature") were used to study the thermodynamics of the bio-adsorption of heavy metals on MDP.The thermodynamic parameters, namely Gibbs free energy change (ΔG , kJ mol −1 ), surface adsorption of entropy change (ΔS, kJ mol −1 K −1 ), and enthalpy change (ΔH, kJ mol −1 ) were calculated using Eqs.(10) 40 and ( 11) 82 and the results are shown in Table 4.The three thermodynamic parameters (ΔG, ΔH, and ΔS) for the bio-adsorption at four temperatures (25-55 °C) are all negative values (Table 4), indicating that the adsorption of Cu 2+ , Ni 2+ , and Zn 2+ on MDP is spontaneous (negative value of ΔG) 83 , exothermic (negative value of ΔH) 84 , and the randomness between the solid/liquid interfaces at the liquid-solid interface is decreased (negative value of ΔS) 85 . (7) ) q e = k ip × t 0.5 + C

Bio-adsorption mechanisms
The adsorption process forms a layer of adsorbate (metal ions) on the surface of adsorbents.Bio-adsorbents, often have a porous structure with various cavities and surface sites (Fig. 4), on which the metal ions can be bonded.The improved pores and cavities on the MDP lead to an increased surface area for adsorption, promoting interaction between the metal ions with the bio-adsorbent 86 .Adsorption of the pollutant onto the adsorbent often includes three steps, namely transportation of the pollutant from aqueous solution to the adsorbent surface, adsorption onto the solid surface, and transport within the adsorbent particle.Kinetic study (Section "Kinetic study") indicates that both the intra-particle diffusion mechanism and the intra-particle diffusion mechanism are not dominant.The heavy metals have a vigorous affinity to the functional groups on the surfaces of the adsorbent 87 .On the surface of agricultural waste, functional groups such as −OH, −COOH, -O-, and −CO-NH-, react with heavy metal ions for their removal from aqueous solution 88 .Therefore, it is very likely that the electrostatic attractions between positively charged metal ions and negatively charged functional groups of bio-adsorbents promote the adsorption capacity 89 .Dispersion of ZnO on the DP surface improves the surface for electro-interaction.ZnO particles are generally found to have a positive value of the zeta potential 90 , which however, can be turned into a negative value because of the appearance of negatively charged centers, e.g., OH − groups on ZnO surface 91 .An illustration of the proposed bio-adsorption mechanisms is shown in Fig. 10.

Reuse of the spent MDP
The reusability of the MDP bio-adsorbent was evaluated by recycling the regenerated MDP for additional 4 times.The used MDP, which was saturated with heavy metals during the bio-adsorption, was regenerated by following a desorption protocol described in Section "Bio-adsorption of heavy metals from aqueous solution and real wastewater".It can be seen from Fig. 11 that the removal percentage is decreased after each recycling.This indicates the irreversible deactivation of the MDP after the adsorption-regeneration cycle, of which the factors are under investigation and will be reported in due course.

Bio-adsorption of a real wastewater using MDP
A real wastewater which was collected from a local electroplating company in Baghdad was applied to evaluate the bio-adsorption performance using the optimized adsorption conditions above, namely MDP particle size of 200 μm, MDP dosing of 2 g L −1 solution , the shaking speed of 300 rpm, adsorption time of 1 h, and the adsorption temperature of 25 °C.The initial and final concentrations of the heavy metals before and after bio-adsorption process, the removal percentage, and the permissible limits are shown in Table 5.It can be seen that the removal efficiencies are higher than 90%, however, the concentration of the heavy metals in the treated water are still higher than the permissible limits 5 for using as the drinking water.This is likely due to the presence of the other heavy metals (such as cadmium and lead, out of this study), which leads to an adsorption competition on the MDF surface.As such, the treated water after the bio-adsorption could only be used for non-drinking water, e.g., agriculture drain.

Conclusion
The present work has shown the significantly improved bio-adsorption efficiency of the date pits (DP)-based bio-adsorbent after modification by loading of ZnO and the followed thermal treatment for the removal of heavy metals (Cu 2+ , Ni 2+ , and Zn 2+ ) in the solution.This is rationalized by the changes in the characteristics (morphology, crystallinity, surface functional groups, and the specific surface area) of the ZnO-modified DP (MDP), compared to those of the raw date pits.At an optimized bio-adsorption condition (e.g., pH of 5, the particle size of 200 μm, the shaking speed of 300 rpm, initial concentration of 90 mg L −1 , MDP dosing of 2 g L −1 solution , adsorption time of 60 min, and the adsorption temperature of 25 °C), a high bio-adsorption efficiency of MDP for the removal of Cu 2+ (97.4%),Ni 2+ (96.7%), and Zn 2+ (90.2%) in the solution was obtained.Irreversible deactivation was observed during the 5 cycles of adsorption-regeneration. Bio-adsorption of a real wastewater showed that the treated water can only be used as for non-drinking water.The isothermal analysis by the Langmuir isotherm model and the Freundlich isotherm model showed that both heterogeneous and homogeneous adsorption sites on MDP surface were involved for the bio-adsorption of heavy metals.According to the Langmuir isotherm model, the maximum bio-adsorption amount of the three heavy metals on MDP follows the order of Cu 2+ (82.4 mg g −1 ) > Ni 2+ (71.9 mg g −1 ) > Zn 2+ (66.3 mg g −1 ).The kinetic study using the pseudo-first-order model, the pseudo-second-order model, and the intra-particle diffusion model suggested that the nature of kinetic adsorption is chemical and the intra-particle diffusion mechanism is not dominant, indicating the multiple bioadsorption mechanisms.The thermodynamic parameters (ΔG, ΔH, and ΔS) pointed out that the bio-adsorption of heavy metals on MDP was spontaneous and exothermic, and the randomness between the solid/liquid interfaces at the liquid-solid interface is decreased.The modification protocol developed in this work is facile and could be a universal treatment of the bio-adsorbents for an improved bio-adsorption performance in terms of the purification of polluted water containing heavy metals.

Figure 1 .
Figure 1.Photos of date pits (DP) and ZnO-modified date pits (MDP) (left) and the steps for the preparation of MDP from DP (right).

Figure 2 .
Figure 2. FT-IR spectra of (a) DP, (b) fresh MDP, and (c) spent MDP after simultaneous adsorption of heavy metals (at bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 90 mg L −1 , MDP dosing of 2 g L −1 solution , adsorption time of 180 min, particle size of 200 µm, shaking speed of 300 rpm, and pH of 5).

Figure 3 .
Figure 3. XRD patterns of (a) DP, (b) fresh MDP, and (c) spent MDP after simultaneous adsorption of heavy metals (at bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 90 mg L −1 , MDP dosing of 2 g L −1 solution , adsorption time of 180 min, particle size of 200 µm, shaking speed of 300 rpm, and pH of 5).

Figure 4 .
Figure 4. SEM images of (a) DP, (b) fresh MDP, and (c) spent MDP after simultaneous adsorption of heavy metals (at bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 90 mg L −1 , MDP dosing of 2 g L −1 solution , adsorption time of 180 min, particle size of 200 µm, shaking speed of 300 rpm, and pH of 5).

Figure 5 .
Figure 5.Effect of (a) pH, (b) particle size, and (c) shaking speed on the removal of heavy metals by the bio-adsorption on MDP.(Bio-adsorption conditions: adsorption temperature 25 °C, initial concentration of 50 mg L −1 , MDP dosing of 2 g L −1 solution , adsorption time of 180 min; (a) particle size of 200 µm and shaking speed of 300 rpm; (b) pH of 5 and shaking speed of 300 rpm; (c) pH of 5 and particle size of 200 µm.)

Figure 6 .Figure 7 .
Figure 6.Effect of MDP dosing on the removal of heavy metals by the bio-adsorption on MDP.(Bioadsorption conditions: adsorption temperature 25 °C, initial concentration of 90 mg L −1 , MDP dosing of 0.4-6 g L −1 solution , particle size of 200 µm, pH of 5, shaking speed of 300 rpm, and adsorption time of 60 min.)

Figure 11 .
Figure 11.The removal percentages of Cu 2+ (a), Ni 2+ (b), and Zn 2+ (c) by the bio-adsorption on the fresh and regenerated MDP.(Bio-adsorption conditions: initial concentration of 90 mg L −1 , MDP dosing of 2 g L −1 solution , particle size of 200 µm, pH of 5, shaking speed of 300 rpm, and adsorption time of 60 min.)

Table 2 .
The adsorption capacity of heavy metals on the date pits (DP)-based bio-adsorbent.modified by loading of ZnO followed by calcination at 350 °C, e multi metals contain Cu 2+ , Ni 2+ , and Zn 2+ , and e according to Langmuir isotherm model.Adsorption conditions in the present work: a Heated at 80 °C, b roased at 120 °C, c

Table 3 .
Parameters of various kinetic models for the bio-adsorption of heavy metals on MDP.C

Table 4 .
Thermodynamic analysis for the bio-adsorption of heavy metals on MDP.

Table 5 .
Bio-adsorption of real wastewater using MDP.